There has been developed a circuit for protecting automotive circuits from transient conditions which operates basically like a switch. The circuit is "on" under normal battery conditions (3 v-16 v), but is "off" under all other conditions, such as double normal battery voltage either in forward (correct) or reverse conditions, load dumping of up to at least 60 volts, and positive and negative transients of up to 120 volts with durations of several milliseconds. The circuit includes, as a major element thereof, a PNP lateral transistor to handle the negative transient requirements.
Lateral PNP transistor structures generally used in the power integrated circuit process result in a parasitic P-type MOS (PMOS) transistor between the collector and the base of the PNP transistor which would adversely affect the operations of the circuit. In addition, under normal circuit conditions, the PNP transistor forms an output pass device operating in saturation, which means that the collector-base junction is forward biased. If the emitter is suddenly taken negative by a negative transient, the parasitic PMOS and the PNP transistor would be turned on with the collector acting as an emitter resulting in a reverse flow of current from the output terminal to the input terminal. Therefore, it would be desirable to modify the PNP lateral transistor to eliminate the adverse effect of the parasitic PMOS and to prevent the device from being turned on by a negative transient.